The in-game features immediately accessible will include: 

New Transmogrification Collections System: Players with gear they wish to transmogg will be able to find, sort, and store items without taking up space in their inventories, banks, or their Void Storage. Items compatible with the transmogg system will be found in the Appearances tab in the Collections interface.   Class Updates: Classes will see a change in spells, talents, and new additions added to some arsenals. Furthermore, players will be able to que up in PVP and PVE areas of the game as any role that their class can fulfill. This will allow players to switch roles for different areas of in-game content and change their playstyles however they so choose.

The events leading up to Legion are as follows: 

Burning Legion Invasions: Familiar places throughout Azeroth will face an infestation of demons, turning locations into battlefields.   Demon Hunter Early Access: Players who pre-ordered Legion will be able to play as the newest class, the Demon Hunter, prior to expansion release. Players will begin the game at level 98, experiencing the corrupted starting zone of Mardum.   The Battle of Broken Shore Begins: With the situation of invading Demons escalating, players will begin battling their way through Broken Shore, the cataclysmic core of the demonic invasion.  

World of Warcraft: Legion releases August 30, 2016.